The present invention relates generally to a frequency converter apparatus of a polyphase direct AC type conventionally employed for a variable voltage and variable frequency power source, which is suitable for operating a variable speed AC motor.
In recent years, a variable voltage and variable frequency power source is widely used for providing an AC motor with variable speed control, thereby avoiding various shortcomings of the commutator and brush of a DC motor, and achieving high-performance motor speed control.
When the power capacity of a motor is small, a power source provided for driving the motor is generally formed with a rectifier for converting AC power into DC power, and an inverter for converting the DC power into AC power with a desired voltage and frequency. Such an inverter is made of semiconductor power devices such as transistors, gate turn-off thyristors or the like, which are artificially switched to obtain the desired voltage and frequency.
On the other hand, when the handling power of a motor is relatively large, a direct frequency converter, called "cycloconverter," is often employed. This is because artificial switching for large power semiconductor devices is difficult in practice. With a cycloconverter, an AC output with an optional voltage and frequency can be obtained directly from AC power supply by means of power supply commutation, i.e., by switching the semiconductor devices with the power supply voltage.
Such a direct frequency converter, however, causes undesirable higher harmonics in the AC line of the power supply. The frequency of the higher harmonics vary depending on the frequency of the AC output supplied to a load (e.g., AC motor). This causes certain shortcomings in the conventional frequency converter.
Generally speaking, a high-power direct frequency converter has a polyphase input and polyphase output, as shown in FIG. 1, wherein the main circuit portion of a 3-phase input and 3-phase-output direct frequency converter (cycloconverter) is shown. The reference numeral 1 denotes a 3-phase AC power supply, the reference numerals 2, 3 and 4 denote power transformers, the reference numerals 5, 6 and 7 denote direct frequency converters, and the reference numeral 8 denotes an AC motor serving as a load. Each of the direct frequency converters is formed of two sets of 3-phase bridge-connected, controlled rectifiers, one set of which is antiparallel coupled to the other set, so that AC currents containing positive and negative components flow into the load.
The 3-phase bridged-connected, controlled rectifier circuit is also used to control a DC motor. In this case, although the load (motor) current is DC, natural numbered higher harmonics of 5th, 7th, 11th, 13th, - - - (i.e., (6n.+-.1)th harmonics; n=natural number) are mixed in the fundamental frequency of the AC power supply.
The basic operation of a direct frequency converter is substantially the same as that of the above controlled rectifier circuit. The direct frequency converter only differs from the above 3-phase controlled rectifier circuit in that the load current and load voltage are AC and are varied, in the form of a sine wave according to the polarity of the load current, with the alternative switching of the antiparallel-connected 3-phase-bridged-connected, controlled rectifier circuit.
Thus, the AC power supply for the direct frequency converter is subjected to not only the above-mentioned natural numbered higher harmonics, but also to other higher harmonics depending on the load frequency.
Assume that the 3-phase power supply frequency is f.sub.IN, the load frequency is f.sub.OUT, and symbols n and m denote given natural numbers. Then, the frequency f.sub.H of the higher harmonics, depending on the load frequency, may be represented as: EQU f.sub.H =(6n.+-.1)f.sub.IN .+-.m.multidot.f.sub.OUT ( 1)
The above equation holds even in a 3-phase (polyphase) output direct frequency converter, except that some of load frequency dependent higher harmonics of number m are automatically cancelled out.
Each natural numbered higher harmonic has a given fixed frequency. Accordingly, such a higher harmonic can be easily removed by means of a low-pass or band-rejection filter. However, the load frequency dependent higher harmonics cannot be easily removed because the frequency of such harmonics varies with changes in the load frequency. In particular, when a phase-advancing capacitor for improving the power factor is adapted to the power system, and if a reactor or the reactive component of a transformer is coupled in series to the phase-advancing capacitor, resonance due to the presence of the capacitor and reactor often occurs near a frequency 3 or 4 times higher than the power supply frequency. When such a resonance matches a certain load-frequency-dependent higher harmonic, even if the natural numbered higher harmonics themselves bring no problem, the resonance induced by the higher harmonic prominently distorts the waveform of the power supply voltage, thereby spoiling the normal operation of other equipment in the power system.